The spatial distribution of post-fire debris flows in relation to observed rainfall anomalies: Insights from the Dolan Fire, California

A range of flow types can be observed in steep, recently-burned terrain, but predicting the spatial distribution of debris flows resulting from a single storm event remains challenging. Prior studies of landslides and debris flows in unburned study areas suggest that threshold rainfall is well-correlated with local climatology, such that rainfall anomaly (rainfall normalized by local climatology) better predicts the spatial distribution of events than absolute peak rainfall. In this study, we evaluate the degree to which the distribution of post-fire debris flows follows a similar pattern. The Dolan Fire burn scar in Monterey County, California, spans a sharp hydroclimatic gradient and experienced a widespread debris-flow triggering storm, providing an ideal natural experiment for testing the predictive power of rainfall anomaly versus absolute rainfall. We present results from remote and field-based mapping at the Dolan Fire, as well as several other fires in Northern California, to show how the magnitude of debris-flow generating rainfall is well-predicted by rainfall anomaly. We also discuss implications for debris-flow prediction, reconnaissance, and mechanisms, which may prove useful as severe wildfires continue to expand into landscapes with limited observational history of post-fire debris flows.

 Cavagnaro (2023) The spatial distribution of post-fire debris flows in relation to observed rainfall anomalies: Insights from the Dolan Fire, California, USGS Landslide Hazards Seminar, 22 March 2023.